1. Field of the Invention
This invention relates generally to the field of charged particle beam deflection, and more particularly to devices and methods used to compensate for both average stage velocity and stage velocity errors when deflecting a charged particle beam across the surface of a substrate carried on the stage.
2. Description of the Related Art
Almost all electron beam systems require means for deflecting the electron beam(s) across the surface of a substrate. This beam deflection is generally accomplished using either electrostatic or magnetic multipoles which generate electric or magnetic fields transverse to the beam direction, thereby inducing side-ways deflection forces to the electron beam as it passes through these deflection elements. The electrostatic and/or magnetic deflection elements require electronic drive circuits capable of generating precise voltages and/or currents to control the electrostatic and/or magnetic deflectors, respectively.
One important application of electron beams is electron-beam lithography (EBL). Examples of EBL systems include Gaussian-beam raster-scanned systems, single shaped beam systems, and electron projection lithography (EPL) systems using masks. Charged particle beam lithography systems also include focused ion beam systems, masked ion beam lithography (MIBL) systems, etc. EBL is regularly used to write masks and reticles needed for the patterning of integrated circuits (ICs) on semiconductor wafers. Recently, interest is growing in the application of EBL for the direct patterning of ICs on wafers—called electron-beam direct-writing (EBDW). The electron beam is focused onto the wafer surface as either a Gaussian beam or a patterned beam, and the electron beam then exposes a resist, which is next developed to produce the pattern, as is familiar to those skilled in the art. For maximum throughput, a writing method called “write-on-the-fly” is commonly used. In this method, the wafer is supported by a wafer stage, typically having at least two axes of motion (X and Y), and often also having additional Z or Yaw motions, as well. The dimensions of modern ICs are now in the 10's of nm range, thus the patterning of ICs necessarily requires very precise positioning of the electron beam being used to write these patterns. Write-on-the-fly requires the wafer to move continuously under the electron beam(s). In most electron beam systems to date, a single writing beam was employed. Recent EBL systems employ multiple electron beams writing simultaneously on the same wafer to increase throughput.
During the write-on-the-fly EBL process, the wafer typically moves in a serpentine pattern, back and forth in a raster pattern. While the wafer is moving, for example parallel to the Y-axis, the beam is deflected along the X-axis to write patterns within a “stripe” which may extend across the entire wafer in a single beam system, or which may be smaller (e.g., 30 mm) in a multiple-beam EBL system. Generally the stage motors are very precisely controlled to move the stage at a pre-determined speed (usually constant). A number of laser interferometers are commonly used to measure the stage position to a resolution <0.1 nm. In EBL systems, the stage position measurements may be used to generate corrective signals for the beam deflectors to enable the electron beam to be correctly positioned on the wafer to accuracies <1 nm, even though the stage mechanical positioning errors may be in excess of 1 μm.
Since the wafer stage control is very precise, it is almost always the case that the stage velocity is held to within a small percentage of the nominal value (typically <1%). One commonly-used approach is to use the stage position data from the laser interferometers to generate a beam deflection signal, which will thus allow the beam to be positioned on the wafer independent of wafer motion. The use of laser interferometers to measure the stage position is described in U.S. Pat. No. 6,355,994 B1, issued Mar. 12, 2002, incorporated herein by reference.
The disadvantage of this simple approach is that very high bandwidth is required to track the stage motion using the laser interferometer data. This can be seen from the fact that at 30 mm/s stage velocity, the stage will move 0.5 nm every 16.67 ns. If 0.5 nm is the maximum acceptable pattern location error, then the beam deflection must update the beam deflection data no less frequently than every 16.67 ns (60 MHz rate). There is a need for a beam deflection system that can allow for high resolution beam placement, without the cost and difficulty of very high bandwidth data processing.